Coated anode material and method of preparing the same

ABSTRACT

The present application provides a coated anode material and a method of preparing the same. The coated anode material has a core-shell structure, wherein the core-shell structure includes an inert core and a shell coated on the inert core, the shell comprises an anode active material, and the inert core comprises a non-active material. In the coated anode material, the anode active material of the shell is distributed over the non-active material of the inert core, and the coated anode material can overcome the volume change problem of silicon particles during lithium insertion/deinsertion to a certain extent and obtain a better cycle performance and rate performance.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. applicationSer. No. 15/207,790, filed on Jul. 12, 2016, which claims priority toChinese patent application No. 201510621549.7, filed on Sep. 25, 2015.The entire disclosure of the above-identified application, including thespecification, drawings and claims are incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

The present application relates to an anode material for a lithium ionbattery and a method of preparing the same, and more particularly to acoated anode material for a lithium ion battery and a method ofpreparing the same.

BACKGROUND OF THE INVENTION

Lithium ion batteries (hereinafter referred as “LIB”) are widely used invarious kinds of electric appliances, and also used as power energy ofelectric vehicles, due to the LIB having advantages of higher operatingvoltage, higher energy density, stable discharge potential, low selfdischarge, long cycle life, no memory effect and no pollution.

When LIB is used as power energy of electric vehicles, the requirementof power energy for LIB is much higher than portable electricappliances. In addition to improve cathode material, electrolyte andseparator, the improvement of anode material is also important.Selection of an anode active material is a key factor that affects theperformance of LIB. The anode active material used in existingcommercial LIBs is mainly graphite, which has a low insertion potentialand an excellent lithium insertion/deinsertion performance. Thus, thegraphite is a good anode active material for LIB. The capacity ofinsertion/deinsertion of lithium ions in graphite can be carried outaccording to chemometry of LiC₆, the theoretical capacity of graphite isreached to 372 mAh/g, but the practical capacity of graphite isgenerally 330 mAh/g, which is quite close to its theoretical capacity.Thus, it is difficult to further improve the capacity of graphite.

The low capacity of carbon-based anode active material (e.g., graphite)restricts the energy density of LIB. Therefore, some non-carbon anodeactive materials have attracted attention by the industry because ofhigher energy density. Among them, silicon is a potential anode activematerial for the upgrading of graphite, wherein the nominal capacity ofsilicon can be reached to 4200 mAh/g, which is much higher thangraphite. Also, the voltage plateau of silicon is higher than graphite,therefore it is not easy to arise lithium plating during charging andwith better safety performance as well. However, the cycle performanceof silicon is not good enough, and the volume change of silicon duringlithium insertion/deinsertion is huge and can be reached to 300%. Thevolume change effect may separate the anode active material from thecurrent collector. Also, silicon by itself is prone to chalking,resulting in a decline in battery performance. In addition, silicon is asemiconductor material, its conductivity is low.

In order to overcome the disadvantages of silicon-based anode activematerial, a lot of works have been done by researches on silicon. Forexample, the silicon material is processed to nanoparticles silicon,porous silicon, or coated silicon, wherein coating examples typicallyinclude coating carbon on silicon or coating inert material on silicon.When the silicon is coated to form a composite material, silicon is themain part of the composite material, the outer layer coated on thesilicon can be used to buffer volume expansion and increase electrontransport ability. By coating a layer of carbon on outer surfaces ofsilicon nanoparticles, a silicon carbon composite material having a coreshell structure is obtained. If the size of silicon particles is atnanoscale, the volume change effect is small, and the existence of alayer of carbon on the silicon will decrease direct contact between thesilicon particles and the electrolyte and improve electron transportcapability among the silicon particles, to enhance cycling stability ofthe whole anode electrode.

Chinese patent application No. 200510119964.9 discloses a siliconcomposite consisting of silicon particles whose surface is at leastpartially coated with a layer of silicon carbide. The size of thesilicon particles is in the range of 50 nm to 50 μm, and an outersurface of the silicon particles is sintered at least partially to forma layer of silicon carbide. By means of coating the silicon particleswith a layer of the silicon carbide, the initial efficiency and thecycling stability are improved, the volume change during charging anddischarging is reduced, making the silicon composite more suitable forLIB anode active material.

Currently, silicon coating is mainly realized by coating a layer ofinert material or carbon material on outer surfaces of siliconparticles. The outer coating layer can control the volume change effectof silicon during charging and discharging. Thus, the problems of greatvolume change during charging and discharging and easily being chalkingcan be solved, to a certain extent, by coating the silicon particles.However, because silicon as an anode active material is coated andcovered by carbon material or inert material, lithium ions have to passthrough the outer coating layer firstly and then embed into the anodeactive material (i.e., the silicon) in the process of lithium insertion.Therefore, the outer coating layer outside the silicon will affect therate capability of the anode material.

SUMMARY OF THE INVENTION

In one aspect, the present application provides a coated anode materialhaving a core-shell structure. The core-shell structure includes aninert core and a shell coated on the inert core. The shell includes ananode active material, and the inert core includes a non-activematerial. In the present application, the non-active material of theinert core has a specific capacity of less than 50 mAh/g. Preferably,the anode active material of the shell has a specific capacity of morethan 500 mAh/g; more preferably, the anode active material of the shellhas a specific capacity of more than 800 mAh/g.

In an embodiment of the present application, the non-active material ofthe inert core is at least one selected from the group consisting ofsilicon carbide, tungsten carbide, titanium carbide, boron carbide,chromium carbide, silicon nitride, aluminium nitride, titanium nitride,zirconium nitride, chromium nitride, barium titanate, aluminiumfluoride, titanium boride, copper powder, barium sulfate, and calciumcarbonate.

In an embodiment of the present application, the anode active materialof the shell is silicon, and the non-active material of the inert coreis silicon carbide.

Silicon carbide is a covalent bonding compound with a crystal latticetightly bonded together. Crystalline silicon carbide is exclusive tolithium ions, and basically there is no lithium intercalation to siliconcarbide. Therefore, silicon carbide cannot be used as anode activematerial alone. Silicon carbide has the advantages of high strength,high flexibility, good resistance to temperature, and no chalking. Thesilicon carbide can be used as anode active material when the siliconcarbide is made into nanowires, in which case the silicon carbide willhave a larger specific surface area with many silicon atoms exposed onthe outer surfaces of the nanowires, such that lithium ions can beembedded between the nanowires of the silicon carbide to obtain a higherlithium intercalation capacity, which can be reached to 876 mAh/g. Inthe present application, the silicon carbide used in constituting theinert core has a covalent bonding structure, with no or very low lithiuminsertion/deinsertion ability, and it can not be used alone as anodeactive material.

In an embodiment of the present application, the inert core has anaverage particle size of 5 nm˜200 nm; preferably, the average particlesize of the inert core is in the range of 10 nm˜100 nm; more preferably,the average particle size of the inert core is in the range of 20 nm˜40nm.

In an embodiment of the present application, the shell has a thicknessof 2 nm˜50 nm; preferably, the thickness of the shell is in the range of3 nm˜10 nm.

In another aspect, the present application provides a method ofpreparing the coated anode material, the method comprising the followingsteps:

(a) dissolving naphthaline and sodium in an organic solvent, dispersinga non-active material in the organic solvent, and adding asilicon-containing compound into the organic solvent for reaction;

(b) washing and drying the product obtained from step (a), and then heattreating the product in an atmosphere of inert gas.

As for step (a), in a first embodiment, the naphthaline is firstlydissolved in the organic solvent, then the non-active material is addedand dispersed in the organic solvent, then the sodium is added anddissolved in the organic solvent, and then the silicon-containingcompound is added into the organic solvent for reaction.

As for step (a), in a second embodiment, a sodium naphthaline solutionis firstly prepared by dissolving the naphthaline and the sodium in theorganic solvent, then the non-active material is added and dispersed inthe sodium naphthaline solution, and then the silicon-containingcompound is added into the sodium naphthaline solution for reaction.

As for step (a), in a third embodiment, the silicon-containing compoundand the non-active material are dispersed in the organic solvent, asodium naphthaline solution is prepared by dissolving naphthaline andsodium in a solvent, and the sodium naphthaline solution is then droppedinto the organic solvent which is dispersed with the silicon-containingcompound and the non-active material.

In an embodiment of the present application, the organic solvent is atleast one selected from the group consisting of ethylene glycol dimethylether, tetrahydrofuran, diethyl ether, 1,4-dioxane, benzol,methylbenzene, dimethylbenzene, ethyl acetate, n-hexane, cyclohexane,and carbonic ester.

In an embodiment of the present application, the non-active material isat least one selected from the group consisting of silicon carbide,tungsten carbide, titanium carbide, boron carbide, chromium carbide,silicon nitride, aluminium nitride, titanium nitride, zirconium nitride,chromium nitride, barium titanate, aluminium fluoride, titanium boride,copper powder, barium sulfate, and calcium carbonate.

In an embodiment of the present application, the silicon-containingcompound is silicon tetrachloride.

In an embodiment of the present application, the inert gas is at leastone selected from the group consisting of nitrogen, hydrogen, argon, andhelium.

In an embodiment of the present application, the heat treatment ispreformed at a temperature between 200° C. and 1000° C. for a periodbetween 1 h and 20 h; preferably, the heat treatment is performed at atemperature between 300° C. and 700° C. for a period between 2 h and 10h.

In an embodiment, the method of preparing the coated anode materialcomprises the following step:

(a) dissolving naphthaline in an organic solvent of ethylene glycoldimethyl ether, adding and dispersing silicon carbide in the organicsolvent, adding and dissolving sodium in the organic solvent, and thenadding silicon tetrachloride into the organic solvent for reaction;

(b) washing and drying the product obtained from step (a), and then heatheating the product at a temperature between 600° C. and 800° C. for aperiod between 2 h and 5 h in an atmosphere of argon.

In a further aspect, the present application provides a lithium ionbattery (LIB) which comprises the above coated anode material.

Usually, traditional coated anode material is a composite anodematerial, which has a core-shell structure with an anode active material(e.g., silicon) as the core and a carbon material or an inert materialas the shell coated on the outer surface of the core. However, thepresent application is different from the traditional coating method. Inthe present application, the active material of silicon is coated on aninert core which has the advantages of high strength, high flexibility,good tolerance to temperature, and no chalking. The coated anodematerial of the present application can overcome the volume change ofsilicon particles during charging and discharging to a certain extentand obtain a better cycle performance and rate performance. On the otherhand, since the active material of silicon is distributed over the outersurface of the coated anode material, the lithium ions do not have topass through any coating layer in the process of lithiuminsertion/deinsertion, so that the rate capability of the anode activematerial is not affected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a SEM photograph of a coated anode material prepared inembodiment 1;

FIG. 2 shows an XRD pattern of a coated anode material prepared inembodiment 1;

FIG. 3 is a graph showing the performance test result of a coin cellprepared in embodiment 1;

FIG. 4 shows a SEM photograph of a coated anode material prepared inembodiment 2; and

FIG. 5 is a graph showing the performance test result of a coin cellprepared in embodiment 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present application will now be described more specifically withreference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this application arepresented herein for purpose of illustration and description only. It isnot intended to be exhaustive or to be limited to the precise formdisclosed.

Embodiment 1

Preparing coated anode material: referring to FIGS. 1-2, dissolving 38.4g naphthaline in 200 ml ethylene glycol dimethyl ether, adding anddispersing 2.0 g silicon carbide (SiC) by ultrasonic dispersion methodfor 1 h, adding 8.36 g sodium and stirring overnight until the sodium isdissolved to form a dark green solution. Then, adding 12.18 g silicontetrachloride for reaction and stirring for 2 h. Thereafter, filteringthe obtained product, washing it three times by cyclohexane, and dryingit. Then, heat treating the product for 3 h under an atmosphere of argonat a temperature of 600° C. As a result, a coated anode material with acore-shell structure is prepared. The core-shell structure includes aninert core and a shell coated on the inert core, wherein the inert coreincludes a non-active material of silicon carbide (SiC), and the shellincludes an anode active material of silicon.

Preparing coin cell: providing 0.1 g the coated anode material asprepared above, 0.1 g conductive carbon black, and 3.333 g PVDF solution(in the NMP solvent, PVDF accounts for 3% by weight), then mixing themfor 10 min, thereafter coating the anode material on current collectorat a thickness of 125 um, forced air drying, cutting piece, and vacuumdrying. As a result, a coil cell is prepared.

Embodiment 2

Preparing coated anode material: referring to FIG. 4, dissolving 38.4 gnaphthaline in 200 ml ethylene glycol dimethyl ether, adding anddispersing 4.0 g silicon carbide (SiC) by ultrasonic dispersion methodfor 1 h, adding 8.36 g sodium and stirring overnight until the sodium isdissolved to form a dark green solution. Then, adding 12.18 g silicontetrachloride for reaction and stirring for 2 h. Thereafter, filteringthe obtained product, washing it three times by cyclohexane, and dryingit. Then, heat treating the product for 3 h under an atmosphere of argonat a temperature of 600° C. As a result, a coated anode material with acore-shell structure is formed. The core-shell structure includes aninert core and a shell coated on the inert core, wherein the inert coreincludes a non-active material of silicon carbide (SiC), and the shellincludes an anode active material of silicon.

Preparing coin cell: same as embodiment 1.

TABLE 1 Coating Initial discharging thickness (nm) capacity (mAh/g)Mid-voltage (V) embodiment 1 6.65 488 0.38 embodiment 2 3.79 204 0.37

The test results of embodiment 1 and embodiment 2 are shown in table 1.As shown from table 1 and FIG. 3 and FIG. 5, the coated anode materialsobtained by coating a layer of silicon on an outer surface of siliconcarbide (SiC) have a good cycle performance. In the embodiment 1, thecoating thickness of the silicon is 6.65 nm, the specific capacity isstable at 380 mAh/g under the rate of 2 C. In the embodiment 2, thecoating thickness of silicon is 3.79 nm, the specific capacity is stableat 200 mAh/g under the rate of 3 C.

Embodiment 3

Preparing coated anode material: dissolving 38.4 g naphthaline in 200 mltetrahydrofuran, adding and dispersing 2.0 g silicon carbide (SiC) byultrasonic dispersion method for 1 h, adding 8.36 g sodium and stirringovernight until the sodium is dissolved to form a dark green solution.Then, adding 12.18 g silicon tetrachloride for reaction and stirring for2 h. Thereafter, filtering the obtained product, washing it three timesby cyclohexane, and drying it. Then, heat treating the product for 10 hunder an atmosphere of nitrogen at a temperature of 300° C. As a result,a coated anode material with a core-shell structure is formed. Thecore-shell structure includes an inert core and a shell coated on theinert core, wherein the inert core includes a non-active material ofsilicon carbide (SiC), and the shell includes an anode active materialof silicon.

Embodiment 4

Preparing coated anode material: dissolving 38.4 g naphthaline in 200 mltetrahydrofuran, adding and dispersing 2.0 g silicon nitride (Si₃N₄) byultrasonic dispersion method for 1 h, adding 8.36 g sodium and stirringovernight until the sodium is dissolved to form a dark green solution.Then, adding 12.18 g silicon tetrachloride for reaction and stirring for2 h. Thereafter, filtering the obtained product, washing it three timesby cyclohexane, and drying it. Then, heat treating the product for 1 hunder an atmosphere of nitrogen at a temperature of 800° C. As a result,a coated anode material with a core-shell structure is formed. Thecore-shell structure includes an inert core and a shell coated on theinert core, wherein the inert core includes a non-active material ofsilicon nitride (Si₃N₄), and the shell includes an anode active materialof silicon.

What is claimed is:
 1. A method of preparing a coated anode material,comprising steps: (a) dissolving naphthaline and sodium in an organicsolvent, dispersing a non-active material in the organic solvent, andadding a silicon-containing compound into the organic solvent forperforming a reaction to obtain a product, wherein the non-activematerial is crystalline silicon carbide or silicon nitride; and (b)washing and drying the product obtained from step (a), and then heattreating the product in an atmosphere of inert gas, to obtain a coatedanode material having a core-shell structure, wherein the core-shellstructure comprises an inert core being in the form of a particle and ashell coated on the inert core, wherein the shell is silicon, and theinert core is silicon carbide or silicon nitride.
 2. The method of claim1, wherein in step (a), the naphthaline is firstly dissolved in theorganic solvent, then the non-active material is added and dispersed inthe organic solvent, then the sodium is added and dissolved in theorganic solvent, and then the silicon-containing compound is added intothe organic solvent for reaction.
 3. The method of claim 1, wherein instep (a), a sodium naphthaline solution is firstly prepared bydissolving the naphthaline and the sodium in the organic solvent, thenthe non-active material is added and dispersed in the sodium naphthalinesolution, and then the silicon-containing compound is added into thesodium naphthaline solution for reaction.
 4. The method of claim 1,wherein in step (a), the silicon-containing compound and the non-activematerial are dispersed in the organic solvent, a sodium naphthalinesolution is prepared by dissolving naphthaline and sodium in a solvent,and the sodium naphthaline solution is then dropped into the organicsolvent which is dispersed with the silicon-containing compound and thenon-active material.
 5. The method of claim 1, wherein the organicsolvent is at least one selected from the group consisting of ethyleneglycol dimethyl ether, tetrahydrofuran, diethyl ether, 1,4-dioxane,benzene, toluene, xylene, ethyl acetate, n-hexane, cyclohexane, andcarbonic ester.
 6. The method of claim 1, wherein the silicon-containingcompound is silicon tetrachloride.
 7. The method of claim 1, wherein theheat treatment is performed at a temperature between 200° C. and 1000°C. for a period between 1 h and 20 h.